/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.lucene.search.suggest.analyzing;

// TODO
//   - test w/ syns
//   - add pruning of low-freq ngrams?

import static org.apache.lucene.util.fst.FST.readMetadata;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.AnalyzerWrapper;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.shingle.ShingleFilter;
import org.apache.lucene.analysis.tokenattributes.OffsetAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionLengthAttribute;
import org.apache.lucene.analysis.tokenattributes.TermToBytesRefAttribute;
import org.apache.lucene.codecs.CodecUtil;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.FieldType;
import org.apache.lucene.document.TextField;
import org.apache.lucene.index.DirectoryReader;
import org.apache.lucene.index.IndexOptions;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.MultiTerms;
import org.apache.lucene.index.Terms;
import org.apache.lucene.index.TermsEnum;
import org.apache.lucene.search.suggest.InputIterator;
import org.apache.lucene.search.suggest.Lookup;
import org.apache.lucene.store.DataInput;
import org.apache.lucene.store.DataOutput;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.util.Accountable;
import org.apache.lucene.util.Accountables;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.BytesRefBuilder;
import org.apache.lucene.util.CharsRefBuilder;
import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.IntsRef;
import org.apache.lucene.util.IntsRefBuilder;
import org.apache.lucene.util.fst.FST;
import org.apache.lucene.util.fst.FST.Arc;
import org.apache.lucene.util.fst.FST.BytesReader;
import org.apache.lucene.util.fst.FSTCompiler;
import org.apache.lucene.util.fst.Outputs;
import org.apache.lucene.util.fst.PositiveIntOutputs;
import org.apache.lucene.util.fst.Util;
import org.apache.lucene.util.fst.Util.Result;
import org.apache.lucene.util.fst.Util.TopResults;

// import java.io.PrintWriter;

/**
 * Builds an ngram model from the text sent to {@link #build} and predicts based on the last grams-1
 * tokens in the request sent to {@link #lookup}. This tries to handle the "long tail" of
 * suggestions for when the incoming query is a never before seen query string.
 *
 * <p>Likely this suggester would only be used as a fallback, when the primary suggester fails to
 * find any suggestions.
 *
 * <p>Note that the weight for each suggestion is unused, and the suggestions are the analyzed forms
 * (so your analysis process should normally be very "light").
 *
 * <p>This uses the stupid backoff language model to smooth scores across ngram models; see "Large
 * language models in machine translation",
 * http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.76.1126 for details.
 *
 * <p>From {@link #lookup}, the key of each result is the ngram token; the value is Long.MAX_VALUE *
 * score (fixed point, cast to long). Divide by Long.MAX_VALUE to get the score back, which ranges
 * from 0.0 to 1.0.
 *
 * <p>onlyMorePopular is unused.
 *
 * @lucene.experimental
 */
public class FreeTextSuggester extends Lookup {

  /** Codec name used in the header for the saved model. */
  public static final String CODEC_NAME = "freetextsuggest";

  /** Initial version of the saved model file format. */
  public static final int VERSION_START = 0;

  /** Current version of the saved model file format. */
  public static final int VERSION_CURRENT = VERSION_START;

  /** By default we use a bigram model. */
  public static final int DEFAULT_GRAMS = 2;

  // In general this could vary with gram, but the
  // original paper seems to use this constant:
  /**
   * The constant used for backoff smoothing; during lookup, this means that if a given trigram did
   * not occur, and we backoff to the bigram, the overall score will be 0.4 times what the bigram
   * model would have assigned.
   */
  public static final double ALPHA = 0.4;

  /** Holds 1gram, 2gram, 3gram models as a single FST. */
  private FST<Long> fst;

  /** Analyzer that will be used for analyzing suggestions at index time. */
  private final Analyzer indexAnalyzer;

  private long totTokens;

  /** Analyzer that will be used for analyzing suggestions at query time. */
  private final Analyzer queryAnalyzer;

  // 2 = bigram, 3 = trigram
  private final int grams;

  private final byte separator;

  /** Number of entries the lookup was built with */
  private volatile long count = 0;

  /**
   * The default character used to join multiple tokens into a single ngram token. The input tokens
   * produced by the analyzer must not contain this character.
   */
  public static final byte DEFAULT_SEPARATOR = 0x1e;

  /**
   * Instantiate, using the provided analyzer for both indexing and lookup, using bigram model by
   * default.
   */
  public FreeTextSuggester(Analyzer analyzer) {
    this(analyzer, analyzer, DEFAULT_GRAMS);
  }

  /**
   * Instantiate, using the provided indexing and lookup analyzers, using bigram model by default.
   */
  public FreeTextSuggester(Analyzer indexAnalyzer, Analyzer queryAnalyzer) {
    this(indexAnalyzer, queryAnalyzer, DEFAULT_GRAMS);
  }

  /**
   * Instantiate, using the provided indexing and lookup analyzers, with the specified model (2 =
   * bigram, 3 = trigram, etc.).
   */
  public FreeTextSuggester(Analyzer indexAnalyzer, Analyzer queryAnalyzer, int grams) {
    this(indexAnalyzer, queryAnalyzer, grams, DEFAULT_SEPARATOR);
  }

  /**
   * Instantiate, using the provided indexing and lookup analyzers, and specified model (2 = bigram,
   * 3 = trigram ,etc.). The separator is passed to {@link ShingleFilter#setTokenSeparator} to join
   * multiple tokens into a single ngram token; it must be an ascii (7-bit-clean) byte. No input
   * tokens should have this byte, otherwise {@code IllegalArgumentException} is thrown.
   */
  public FreeTextSuggester(
      Analyzer indexAnalyzer, Analyzer queryAnalyzer, int grams, byte separator) {
    this.grams = grams;
    this.indexAnalyzer = addShingles(indexAnalyzer);
    this.queryAnalyzer = addShingles(queryAnalyzer);
    if (grams < 1) {
      throw new IllegalArgumentException("grams must be >= 1");
    }
    if ((separator & 0x80) != 0) {
      throw new IllegalArgumentException("separator must be simple ascii character");
    }
    this.separator = separator;
  }

  /** Returns byte size of the underlying FST. */
  @Override
  public long ramBytesUsed() {
    if (fst == null) {
      return 0;
    }
    return fst.ramBytesUsed();
  }

  @Override
  public Collection<Accountable> getChildResources() {
    if (fst == null) {
      return Collections.emptyList();
    } else {
      return Collections.singletonList(Accountables.namedAccountable("fst", fst));
    }
  }

  private Analyzer addShingles(final Analyzer other) {
    if (grams == 1) {
      return other;
    } else {
      // TODO: use ShingleAnalyzerWrapper?
      // Tack on ShingleFilter to the end, to generate token ngrams:
      return new AnalyzerWrapper(other.getReuseStrategy()) {
        @Override
        protected Analyzer getWrappedAnalyzer(String fieldName) {
          return other;
        }

        @Override
        protected TokenStreamComponents wrapComponents(
            String fieldName, TokenStreamComponents components) {
          ShingleFilter shingles = new ShingleFilter(components.getTokenStream(), 2, grams);
          shingles.setTokenSeparator(Character.toString((char) separator));
          return new TokenStreamComponents(components.getSource(), shingles);
        }
      };
    }
  }

  @Override
  public void build(InputIterator iterator) throws IOException {
    build(iterator, IndexWriterConfig.DEFAULT_RAM_BUFFER_SIZE_MB);
  }

  /**
   * Build the suggest index, using up to the specified amount of temporary RAM while building. Note
   * that the weights for the suggestions are ignored.
   */
  public void build(InputIterator iterator, double ramBufferSizeMB) throws IOException {
    if (iterator.hasPayloads()) {
      throw new IllegalArgumentException("this suggester doesn't support payloads");
    }
    if (iterator.hasContexts()) {
      throw new IllegalArgumentException("this suggester doesn't support contexts");
    }

    IndexWriterConfig iwc = new IndexWriterConfig(indexAnalyzer);
    iwc.setOpenMode(IndexWriterConfig.OpenMode.CREATE);
    iwc.setRAMBufferSizeMB(ramBufferSizeMB);

    String prefix = getClass().getSimpleName();
    Path tempIndexPath = Files.createTempDirectory(prefix + ".index.");
    try (Directory dir = FSDirectory.open(tempIndexPath);
        IndexWriter writer = new IndexWriter(dir, iwc)) {

      FieldType ft = new FieldType(TextField.TYPE_NOT_STORED);
      // TODO: if only we had IndexOptions.TERMS_ONLY...
      ft.setIndexOptions(IndexOptions.DOCS_AND_FREQS);
      ft.setOmitNorms(true);
      ft.freeze();

      Document doc = new Document();
      Field field = new Field("body", "", ft);
      doc.add(field);

      totTokens = 0;

      long newCount = 0;
      while (true) {
        BytesRef surfaceForm = iterator.next();
        if (surfaceForm == null) {
          break;
        }
        field.setStringValue(surfaceForm.utf8ToString());
        writer.addDocument(doc);
        newCount++;
      }
      try (IndexReader reader = DirectoryReader.open(writer)) {

        Terms terms = MultiTerms.getTerms(reader, "body");
        if (terms == null) {
          throw new IllegalArgumentException("need at least one suggestion");
        }

        // Move all ngrams into an FST:
        TermsEnum termsEnum = terms.iterator();

        Outputs<Long> outputs = PositiveIntOutputs.getSingleton();
        FSTCompiler<Long> fstCompiler =
            new FSTCompiler.Builder<>(FST.INPUT_TYPE.BYTE1, outputs).build();

        IntsRefBuilder scratchInts = new IntsRefBuilder();
        while (true) {
          BytesRef term = termsEnum.next();
          if (term == null) {
            break;
          }
          int ngramCount = countGrams(term);
          if (ngramCount > grams) {
            throw new IllegalArgumentException(
                "tokens must not contain separator byte; got token="
                    + term
                    + " but gramCount="
                    + ngramCount
                    + ", which is greater than expected max ngram size="
                    + grams);
          }
          if (ngramCount == 1) {
            totTokens += termsEnum.totalTermFreq();
          }

          fstCompiler.add(
              Util.toIntsRef(term, scratchInts), encodeWeight(termsEnum.totalTermFreq()));
        }

        final FST<Long> newFst =
            FST.fromFSTReader(fstCompiler.compile(), fstCompiler.getFSTReader());
        if (newFst == null) {
          throw new IllegalArgumentException("need at least one suggestion");
        }
        fst = newFst;
        count = newCount;

        // System.out.println("FST: " + fst.getNodeCount() + " nodes");

        /*
        PrintWriter pw = new PrintWriter("/x/tmp/out.dot");
        Util.toDot(fst, pw, true, true);
        pw.close();
        */

        // Writer was only temporary, to count up bigrams,
        // which we transferred to the FST, so now we
        // rollback:
        writer.rollback();
      }
    } finally {
      IOUtils.rm(tempIndexPath);
    }
  }

  @Override
  public boolean store(DataOutput output) throws IOException {
    CodecUtil.writeHeader(output, CODEC_NAME, VERSION_CURRENT);
    output.writeVLong(count);
    output.writeByte(separator);
    output.writeVInt(grams);
    output.writeVLong(totTokens);
    fst.save(output, output);
    return true;
  }

  @Override
  public boolean load(DataInput input) throws IOException {
    CodecUtil.checkHeader(input, CODEC_NAME, VERSION_START, VERSION_START);
    count = input.readVLong();
    byte separatorOrig = input.readByte();
    if (separatorOrig != separator) {
      throw new IllegalStateException(
          "separator="
              + separator
              + " is incorrect: original model was built with separator="
              + separatorOrig);
    }
    int gramsOrig = input.readVInt();
    if (gramsOrig != grams) {
      throw new IllegalStateException(
          "grams=" + grams + " is incorrect: original model was built with grams=" + gramsOrig);
    }
    totTokens = input.readVLong();

    fst = new FST<>(readMetadata(input, PositiveIntOutputs.getSingleton()), input);

    return true;
  }

  @Override
  public List<LookupResult> lookup(
      final CharSequence key, /* ignored */ boolean onlyMorePopular, int num) {
    return lookup(key, null, onlyMorePopular, num);
  }

  /** Lookup, without any context. */
  public List<LookupResult> lookup(final CharSequence key, int num) {
    return lookup(key, null, true, num);
  }

  @Override
  public List<LookupResult> lookup(
      final CharSequence key,
      Set<BytesRef> contexts, /* ignored */
      boolean onlyMorePopular,
      int num) {
    try {
      return lookup(key, contexts, num);
    } catch (IOException ioe) {
      // bogus:
      throw new RuntimeException(ioe);
    }
  }

  @Override
  public long getCount() {
    return count;
  }

  private int countGrams(BytesRef token) {
    int count = 1;
    for (int i = 0; i < token.length; i++) {
      if (token.bytes[token.offset + i] == separator) {
        count++;
      }
    }

    return count;
  }

  /** Retrieve suggestions. */
  public List<LookupResult> lookup(final CharSequence key, Set<BytesRef> contexts, int num)
      throws IOException {
    if (contexts != null) {
      throw new IllegalArgumentException("this suggester doesn't support contexts");
    }
    if (fst == null) {
      throw new IllegalStateException("Lookup not supported at this time");
    }

    try (TokenStream ts = queryAnalyzer.tokenStream("", key.toString())) {
      TermToBytesRefAttribute termBytesAtt = ts.addAttribute(TermToBytesRefAttribute.class);
      OffsetAttribute offsetAtt = ts.addAttribute(OffsetAttribute.class);
      PositionLengthAttribute posLenAtt = ts.addAttribute(PositionLengthAttribute.class);
      PositionIncrementAttribute posIncAtt = ts.addAttribute(PositionIncrementAttribute.class);
      ts.reset();

      BytesRefBuilder[] lastTokens = new BytesRefBuilder[grams];
      // System.out.println("lookup: key='" + key + "'");

      // Run full analysis, but save only the
      // last 1gram, last 2gram, etc.:
      int maxEndOffset = -1;
      boolean sawRealToken = false;
      while (ts.incrementToken()) {
        BytesRef tokenBytes = termBytesAtt.getBytesRef();
        sawRealToken |= tokenBytes.length > 0;
        // TODO: this is somewhat iffy; today, ShingleFilter
        // sets posLen to the gram count; maybe we should make
        // a separate dedicated att for this?
        int gramCount = posLenAtt.getPositionLength();

        assert gramCount <= grams;

        // Safety: make sure the recalculated count "agrees":
        if (countGrams(tokenBytes) != gramCount) {
          throw new IllegalArgumentException(
              "tokens must not contain separator byte; got token="
                  + tokenBytes
                  + " but gramCount="
                  + gramCount
                  + " does not match recalculated count="
                  + countGrams(tokenBytes));
        }
        maxEndOffset = Math.max(maxEndOffset, offsetAtt.endOffset());
        BytesRefBuilder b = new BytesRefBuilder();
        b.append(tokenBytes);
        lastTokens[gramCount - 1] = b;
      }
      ts.end();

      if (!sawRealToken) {
        throw new IllegalArgumentException(
            "no tokens produced by analyzer, or the only tokens were empty strings");
      }

      // Carefully fill last tokens with _ tokens;
      // ShingleFilter appraently won't emit "only hole"
      // tokens:
      int endPosInc = posIncAtt.getPositionIncrement();

      // Note this will also be true if input is the empty
      // string (in which case we saw no tokens and
      // maxEndOffset is still -1), which in fact works out OK
      // because we fill the unigram with an empty BytesRef
      // below:
      boolean lastTokenEnded = offsetAtt.endOffset() > maxEndOffset || endPosInc > 0;
      // System.out.println("maxEndOffset=" + maxEndOffset + " vs " + offsetAtt.endOffset());

      if (lastTokenEnded) {
        // System.out.println("  lastTokenEnded");
        // If user hit space after the last token, then
        // "upgrade" all tokens.  This way "foo " will suggest
        // all bigrams starting w/ foo, and not any unigrams
        // starting with "foo":
        for (int i = grams - 1; i > 0; i--) {
          BytesRefBuilder token = lastTokens[i - 1];
          if (token == null) {
            continue;
          }
          token.append(separator);
          lastTokens[i] = token;
        }
        lastTokens[0] = new BytesRefBuilder();
      }

      Arc<Long> arc = new Arc<>();

      BytesReader bytesReader = fst.getBytesReader();

      // Try highest order models first, and if they return
      // results, return that; else, fallback:
      double backoff = 1.0;

      List<LookupResult> results = new ArrayList<>(num);

      // We only add a given suffix once, from the highest
      // order model that saw it; for subsequent lower order
      // models we skip it:
      final Set<BytesRef> seen = new HashSet<>();

      for (int gram = grams - 1; gram >= 0; gram--) {
        BytesRefBuilder token = lastTokens[gram];
        // Don't make unigram predictions from empty string:
        if (token == null || (token.length() == 0 && key.length() > 0)) {
          // Input didn't have enough tokens:
          // System.out.println("  gram=" + gram + ": skip: not enough input");
          continue;
        }

        if (endPosInc > 0 && gram <= endPosInc) {
          // Skip hole-only predictions; in theory we
          // shouldn't have to do this, but we'd need to fix
          // ShingleFilter to produce only-hole tokens:
          // System.out.println("  break: only holes now");
          break;
        }

        // System.out.println("try " + (gram+1) + " gram token=" + token.utf8ToString());

        // TODO: we could add fuzziness here
        // match the prefix portion exactly
        // Pair<Long,BytesRef> prefixOutput = null;
        Long prefixOutput = null;
        try {
          prefixOutput = lookupPrefix(fst, bytesReader, token.get(), arc);
        } catch (IOException bogus) {
          throw new RuntimeException(bogus);
        }
        // System.out.println("  prefixOutput=" + prefixOutput);

        if (prefixOutput == null) {
          // This model never saw this prefix, e.g. the
          // trigram model never saw context "purple mushroom"
          backoff *= ALPHA;
          continue;
        }

        // TODO: we could do this division at build time, and
        // bake it into the FST?

        // Denominator for computing scores from current
        // model's predictions:
        long contextCount = totTokens;

        BytesRef lastTokenFragment = null;

        for (int i = token.length() - 1; i >= 0; i--) {
          if (token.byteAt(i) == separator) {
            BytesRef context = new BytesRef(token.bytes(), 0, i);
            Long output = Util.get(fst, Util.toIntsRef(context, new IntsRefBuilder()));
            assert output != null;
            contextCount = decodeWeight(output);
            lastTokenFragment = new BytesRef(token.bytes(), i + 1, token.length() - i - 1);
            break;
          }
        }

        final BytesRefBuilder finalLastToken = new BytesRefBuilder();
        if (lastTokenFragment == null) {
          finalLastToken.copyBytes(token.get());
        } else {
          finalLastToken.copyBytes(lastTokenFragment);
        }

        CharsRefBuilder spare = new CharsRefBuilder();

        // complete top-N
        TopResults<Long> completions = null;
        try {

          // Because we store multiple models in one FST
          // (1gram, 2gram, 3gram), we must restrict the
          // search so that it only considers the current
          // model.  For highest order model, this is not
          // necessary since all completions in the FST
          // must be from this model, but for lower order
          // models we have to filter out the higher order
          // ones:

          // Must do num+seen.size() for queue depth because we may
          // reject up to seen.size() paths in acceptResult():
          Util.TopNSearcher<Long> searcher =
              new Util.TopNSearcher<>(fst, num, num + seen.size(), Comparator.naturalOrder()) {

                BytesRefBuilder scratchBytes = new BytesRefBuilder();

                @Override
                protected void addIfCompetitive(Util.FSTPath<Long> path) {
                  if (path.arc.label() != separator) {
                    // System.out.println("    keep path: " + Util.toBytesRef(path.input, new
                    // BytesRef()).utf8ToString() + "; " + path + "; arc=" + path.arc);
                    super.addIfCompetitive(path);
                  } else {
                    // System.out.println("    prevent path: " + Util.toBytesRef(path.input, new
                    // BytesRef()).utf8ToString() + "; " + path + "; arc=" + path.arc);
                  }
                }

                @Override
                protected boolean acceptResult(IntsRef input, Long output) {
                  Util.toBytesRef(input, scratchBytes);
                  finalLastToken.grow(finalLastToken.length() + scratchBytes.length());
                  int lenSav = finalLastToken.length();
                  finalLastToken.append(scratchBytes);
                  // System.out.println("    accept? input='" + scratchBytes.utf8ToString() + "';
                  // lastToken='" + finalLastToken.utf8ToString() + "'; return " +
                  // (seen.contains(finalLastToken) == false));
                  boolean ret = seen.contains(finalLastToken.get()) == false;

                  finalLastToken.setLength(lenSav);
                  return ret;
                }
              };

          // since this search is initialized with a single start node
          // it is okay to start with an empty input path here
          searcher.addStartPaths(arc, prefixOutput, true, new IntsRefBuilder());

          completions = searcher.search();
          assert completions.isComplete;
        } catch (IOException bogus) {
          throw new RuntimeException(bogus);
        }

        int prefixLength = token.length();

        BytesRefBuilder suffix = new BytesRefBuilder();
        // System.out.println("    " + completions.length + " completions");

        nextCompletion:
        for (Result<Long> completion : completions) {
          token.setLength(prefixLength);
          // append suffix
          Util.toBytesRef(completion.input(), suffix);
          token.append(suffix);

          // System.out.println("    completion " + token.utf8ToString());

          // Skip this path if a higher-order model already
          // saw/predicted its last token:
          BytesRef lastToken = token.get();
          for (int i = token.length() - 1; i >= 0; i--) {
            if (token.byteAt(i) == separator) {
              assert token.length() - i - 1 > 0;
              lastToken = new BytesRef(token.bytes(), i + 1, token.length() - i - 1);
              break;
            }
          }
          if (seen.contains(lastToken)) {
            // System.out.println("      skip dup " + lastToken.utf8ToString());
            continue nextCompletion;
          }
          seen.add(BytesRef.deepCopyOf(lastToken));
          spare.copyUTF8Bytes(token.get());
          LookupResult result =
              new LookupResult(
                  spare.toString(),
                  (long)
                      (Long.MAX_VALUE
                          * backoff
                          * ((double) decodeWeight(completion.output()))
                          / contextCount));
          results.add(result);
          assert results.size() == seen.size();
          // System.out.println("  add result=" + result);
        }
        backoff *= ALPHA;
      }

      results.sort(
          (a, b) -> {
            if (a.value > b.value) {
              return -1;
            } else if (a.value < b.value) {
              return 1;
            } else {
              // Tie break by UTF16 sort order:
              return ((String) a.key).compareTo((String) b.key);
            }
          });

      if (results.size() > num) {
        results.subList(num, results.size()).clear();
      }

      return results;
    }
  }

  /** weight -&gt; cost */
  private long encodeWeight(long ngramCount) {
    return Long.MAX_VALUE - ngramCount;
  }

  /** cost -&gt; weight */
  // private long decodeWeight(Pair<Long,BytesRef> output) {
  private long decodeWeight(Long output) {
    assert output != null;
    return (int) (Long.MAX_VALUE - output);
  }

  // NOTE: copied from WFSTCompletionLookup & tweaked
  private Long lookupPrefix(
      FST<Long> fst, FST.BytesReader bytesReader, BytesRef scratch, Arc<Long> arc)
      throws /*Bogus*/ IOException {

    Long output = fst.outputs.getNoOutput();

    fst.getFirstArc(arc);

    byte[] bytes = scratch.bytes;
    int pos = scratch.offset;
    int end = pos + scratch.length;
    while (pos < end) {
      if (fst.findTargetArc(bytes[pos++] & 0xff, arc, arc, bytesReader) == null) {
        return null;
      } else {
        output = fst.outputs.add(output, arc.output());
      }
    }

    return output;
  }

  /** Returns the weight associated with an input string, or null if it does not exist. */
  public Object get(CharSequence key) {
    throw new UnsupportedOperationException();
  }
}
